6 LEAF-MINING INSECTS 



In so far as the work of these insects is concerned, we may 

 regard the tissues in the leaf of two sorts, one of which is 

 eaten and one is not. The latter is the epidermis with its 

 cuticle. 1 The other, whether parenchyma or vein, or both, 

 we may for practical purposes designate as mesophyll. 



THE INSECTS 



Leaf-miners are all larvae. No adult insects have been 

 able to establish themselves in such a habitat. It is only 

 the worm-like, quick-growing young of those groups of 

 insects which have complete metamorphosis with both 

 larval and pupal stages in their life cycle, those that feed 

 extensively on plants, and are very small in size, that have 

 become leaf-miners. They are the larvae of four great 

 groups, which, named in the order of their importance are: 



1. Caterpillars or moth larvae; order Lepidoptera 



2. Grubs or beetle larvae; order Coleoptera 



3. Maggots and other two-winged fly larvae; order Diptera 



4. Sawfly larvae; order Hymenoptera. 



These are the same four orders that everywhere are the 

 world's keenest competitors for food, and that make up the 

 bulk of the animal population. 



The pressure for room and for sustenance has been very 

 great and all kinds of shifts for a living have been tried; 

 and these little fellows have shifted into the interior of 

 leaves where the mesophyll provides abundant food, and the 

 epidermis is their shelter. 



The adults of these four orders are very unlike; moths and 

 beetles and true flies, and sawflies. No one would imagine 

 that they could come from larvae that seem so nearly alike. 

 Every one who has studied leaf-miners is at once struck by 

 this similarity of larvae. It is a remarkably good illustra- 

 tion of the moulding power of environment. Within the 

 leaf the conditions are alike for all, and all have been moulded 



1 Tn descriptions sometimes mentioned by either name. 



